For purposes herein, synthesis gas (syngas) is a gas containing carbon monoxide and frequently hydrogen, although term “syngas”, for purposes herein, is also intended to encompass carbon monoxide gas streams that may have little or no hydrogen. Typically, carbon monoxide is present in an amount of at least about 20 volume percent, and the syngas typically contains other components in addition to hydrogen such as carbon dioxide, nitrogen and water vapor. Syngas may derived from various sources, including, but not limited to, gasification of carbonaceous feedstocks such as biomass, landfill gas, coal, natural gas, and petroleum; coke oven gas and gas from other industrial operations such as petroleum refining and steel mill waste gas.
Anaerobic fermentations of carbon monoxide and hydrogen and carbon dioxide have also been proposed and involve the contact of the substrate gas in a liquid, aqueous menstruum with microorganisms capable of generating oxygenated organic compounds such as ethanol, acetic acid, n-propanol and n-butanol. For instance, the theoretical equations for the conversion of carbon monoxide and hydrogen to ethanol are:6CO+3H2O.C2H5OH+4CO2 6H2+2CO2.C2H5OH+3H2O.As can be seen, the conversion of carbon monoxide results in the generation of carbon dioxide. The conversion of hydrogen involves the consumption of hydrogen and carbon dioxide, and this conversion is sometimes referred to as the H2/CO2 conversion.
Syngas typically contains components other than carbon monoxide, hydrogen, carbon dioxide, nitrogen, and water vapor. Some of these components such as ammonia, carbonyl sulfide and hydrogen sulfide can be tolerated by the microorganisms, and some may even be able to serve as nutrients or additives useful to the microorganisms. However, some of these components such as tars, benzene, toluene, xylene, ethylene, acetylene, hydrogen cyanide and nitric oxide are typically contained in syngas at concentrations that pose undesirable effects on the microorganism, processing equipment and product quality. For example, tars, which are naphthalene and heavier aromatic compounds, become solid at temperatures used for anaerobic fermentation, and the solids can build-up resulting in operational problems with piping, pumps, instrument probes and valves. Accordingly, the syngas must be subjected to a clean-up (refining) operation to make it suitable for supplying substrate to anaerobic fermentations.
A desire exists to use syngas efficiently both in the fermentation operation to make higher value products and in conserving the syngas values in any cleanup operation. In addition to capital and operating costs for effecting syngas clean-up, consideration must be given to the costs for safe and environmentally acceptable disposal of the removed contaminants. For a syngas to oxygenated organic compound fermentation process to be commercially viable, capital and operating costs must be sufficiently low that it is at least competitive with alternative biomass to oxygenated organic compound processes. For instance, ethanol is currently commercially produced from corn and cane sugar in facilities having name plate capacities of over 100 million gallons per year at sufficiently low costs to be competitive with fossil fuels. Biomass to syngas to oxygenated organic compound fermentation processes face even greater challenges due to the multiple major operations required to convert the biomass to syngas, to cleanup the syngas sufficiently to be used in an anaerobic fermentation, to effect the anaerobic fermentation and then to recover a merchantable product. Moreover, conservation of nutrients, adjuvants and resources such as water is important to provide a commercially attractive process.
Processes are therefore sought to refine and then bioconvert syngas to oxygenated organic compound at low capital and operating costs.